1. Field of Technology
The present invention relates to an image forming apparatus which corrects image distortion caused by skew of a line image sensor, inclination of an original document, and the like.
2. Description of Prior Art
In an apparatus such as a digital copier, the original document is read out in two-dimensions by repeating each line reading-out with the line image sensor, while the original document is moved in a perpendicular direction (sub-scanning direction Y) to the direction of reading-out with the line image sensor (namely main-scanning direction X). In such apparatus, in cases where the reading-out direction (main-scanning direction) and the sub-scanning direction are not aligned in a perpendicular relation (namely being skewing) to each other, a rectangular original document is read out with being distorted into parallelogram (F1 in the drawing) as shown in FIG. 6(a) for example. In the case of electronically correcting such image distortion or inclination, or in the case of correcting the image F1 in FIG. 6(a), into the image F2 in FIG. 6(b), at least the correction of skew (dislocation) in the sub-scanning direction is necessary. In order to execute this skew correction in real time by simultaneously keeping pace with the reading operation of the original document, usually, line memories corresponding to a correction amount A (shaded area in FIG. 6(a)) are necessary. For example, to cope with a distortion of approximately 10 mm (corresponding to 216 lines), memory for 108 lines is required (since shaded area A is a triangle, half of 216 are required).
Meanwhile, in the apparatus such as a digital copier, a system is adopted where the image data obtained by reading-out from an original document is once stored in the memory, and after reading-out from the memory, the image data is applied a certain image processing to output the image. Since the data amount of the image data is very large, for example 200 mega bytes for one page of A3 size original document, such image data is usually compressed and stored in the memory. In a compression system, such as BCT compression where the image data is divided into blocks of 4×4, for example, and compressed by a unit of the block, and JPEG compression are utilized. There is a technology to conduct an image rotation processing by devising the arrangement of the image of each block, in the operation of reading-out and writing-in of image data.
Further, there is a technology where the image data stored in the memory is divided into a block unit having the size and the shape in accordance with the angle of image inclination, and the relative position of each block is shifted by the unit of a pixel, and rearranged on the memory, thus the inclination of the image is corrected. According to this technology in the case of performing the compression and expansion by the unit of block, the above rearrangement is executed (refer to JPA2006-203710 for example).
In the case where the technology of image rotation by changing the arrangement of blocks is applied for the skew correction, since the minimum correction amount is the block size, for example the image of FIG. 13(a) is corrected to be the image of FIG. 13(b), and continuity of the image is lost and image quality is extremely degraded.
According to the technology described in JPA2006-203710, since the relative position of each block is shifted by the unit of one pixel and rearranged on the memory, the minimum correction amount becomes one pixel, however, a smaller amount correction than the one pixel unit is not possible, which causes deterioration of the image quality. Further, since the block size changes according to the angle of inclination, the processing becomes complicated. In cases where the block size is relatively small, the compression rate is lowered, and when the block size becomes large, the image deterioration increases due to irreversible compression. Moreover, since the relative position of each block is shifted and rearranged on the memory, an equivalent memory area to shaded area A in FIG. 6(a) is needed, which raises a problem of increasing cost of the apparatus.
An object of the present invention is to solve the above problems, and to provide an image forming apparatus which can correct image skew with a relatively small required amount of memory, and an image forming apparatus which ensures less image deterioration at the time of skew correction.